The invention relates to a crack indicator for monitoring the insulation, in particular the internal insulation, of gas reservoirs for low boiling, liquefied gases. Low temperature liquids, for example liquefied gases (down to -162.degree. C.) are stored and transported in insulated steel or concrete containers (liquefied gas tanks). The thermal insulation has to meet high safety requirements as a thermal bridge is produced if the insulating layer is cracked or damaged, and liquefied gas flows out. In addition to the evaporation losses, this can lead to super-cooling of the supporting mountings which are possibly not tough at very low temperatures and would thus be damaged (brittle fracture). In liquefied gas tanks which are designed for transportation by sea, even higher requirements are made on the perfect functioning of the insulation.
For this reason, warning systems have been developed which immediately indicate malfunctioning of the tank (first barrier) and insulation.
The Inter-Governmental Maritime Consultative Organization (IMCO) gas tanker code prescribes, for example, a gas detection system which is based on the fact that when liquefied gas passes through the first barrier (tank), it penetrates the inter-barrier space and can be detected therein using a gas detector. For this purpose, the cavities are flushed with nitrogen or purified exhaust gas. The flushing gas is conveyed through a gas detector which reacts specifically on the filling gas and emits a warning signal if a crack occurs in the insulation.
Another crack indicating system also makes use of the gas detection principle, but is based on a pipe work system imbedded in the insulation. The pipe work system is continuously traversed by a flushing gas and communicates, in a similar manner to the device described above, with a gas detector which reacts to the specific filling gas. As known from experience, since the insulating layer is subjected to a high tensile stress, once a crack has occurred, it self-propagates further and tears or at least damages the pipe work system at this point. The evaporated filling gas can then penetrate through the resultant leak in the pipe work system into the pipe and passes to the gas detector which triggers an alarm in the same manner as described above.
In addition to these devices which are based on the principle of leakage indication, electrical warning systems have become known, which are based on an electrical conductor system imbedded in the insulating layer. In this case, the change in the electrical properties in the event of a crack serve to trigger an alarm. Thus, a crack indicator for the internal insulation of a liquefied gas tank which is based on a network of resistance strain gauges imbedded in the insulating layer is described, for example, in German Offenlegungsschrift No. 2 713 007. A crack in the insulation can then be detected due to the change in the strain condition on the respective resistance strain gauge.
From a technical viewpoint, the warning system has to meet the following requirements:
1. The incorporation of a piping system into the insulating layer should not be accompanied by problems and should also not impair the strength of the insulating layer. PA1 2. The warning system must be easy to assemble, including the associated auxiliary devices. PA1 3. It must be possible to check operation before the first filling of the liquefied gas tank and also at any later moment without the need for large modifications. PA1 4. A crack in the insulation should not only be capable of being indicated but also of being located. PA1 5. In the event of damage, it must be possible to repair the insulating layer, in which process the warning system must be shifted back into its previous condition. PA1 1. In the event of damage, the warning gas issuing causes an alarm or an indication irrespective of whether the container is empty or full and irrespective of the type of filling. PA1 2. Increased reliability of crack indication is achieved, particularly if the detection of the warning gas in the dome of the container and the indication of a flow of warning gas in the piping system are combined with each other. PA1 3. The pneumatic display affords the advantage that the test signal can be used to locate the damage. PA1 4. The new crack indication system is composed of tested conventional commercial components. PA1 5. In contrast with earlier methods, flushing gas is not needed. Warning gas is only consumed to a small extent in the event of damage. PA1 6. The strength of the insulating system is not impaired by the imbedded piping system. PA1 7. The operating capacity of the crack indication system can be checked before it is installed, during installation or in use without large modifications. PA1 8. The assembly of the piping system is relatively simple since the piping system is available as a continuous product and in the case of the frequently employed foam insulation, is simply foamed in.
These requirements cannot all be met in the warning systems according to the prior art.